March Meeting Briefs

Buckyballs, Optical Lattices, and Subwavelength Imaging

Buckyballs can be made from elements other than carbon, forming a variety of potentially useful structures. Lai-Sheng Wang of Washington State University has produced hollow sphere-shaped cages made from gold and tin atoms. Wang’s group formed a variety of gold structures, ranging from 2-dimensional structures with 4 to 12 atoms to pyramids made of 20 gold atoms. In between, gold clusters with 16 to 18 atoms formed hollow cages. Wang’s group created the clusters using pulsed laser vaporization, in which a laser vaporizes atoms off a solid gold target. The vaporized atoms condense into clusters of various sizes, as small as a few atoms, which are then sorted by size. Wang used photoelectron spectroscopy to look at the structures of the clusters.

The researchers have also been able to insert other atoms into the hollow centers of the cages, changing the electronic, magnetic, and catalytic properties of the structures.

The gold cages, first produced a couple of years ago, were the first metal buckyball-like structures produced. More recently, Wang reported at the March Meeting that he has produced stable tin icosahedral structures, which he calls “stannashperene.” He has inserted various transition metal elements into these cages as well. While it is too early to develop any specific applications, these structures have interesting properties, and could be potential building blocks for new materials, Wang said in a press conference at the March Meeting.

In a step towards quantum computing and other applications, David Weiss of Penn State reported that he and colleagues have demonstrated a new 3D optical lattice in which they have trapped and manipulated 250 atoms for potential use as qubits.

Optical lattices use a set of crossed laser beams to create an array of sites where atoms can be trapped in potential wells. The researchers load a small trap with cesium atoms, then turn on the lasers. The lattice starts with about six atoms per site; after laser cooling, each site in the lattice is left with either one or zero atoms. The spacing between atoms in their lattice is about 5 microns, which gives enough space between atoms that they can address individual atoms and manipulate the states of individual atoms with lasers. The researchers have imaged 250 individual neutral atoms in this array, and are working on filling in all the vacancies in the lattice. The method could be scaled up to create arrays with thousands of atoms.

They hope to use the trapped atoms as qubits. “They are really isolated perfect quantum systems,” said Weiss. He has proposed a way to use the optical lattice to execute single or two qubit gates for quantum computation. While there are many potential routes to quantum computing, said Weiss, one advantage of neutral atoms in these traps is that they have very weak interactions with the outside world. Optical lattices have other uses as well, Weiss pointed out. “The hottest use for optical lattices is to simulate condensed matter systems,” he said.

Images with subwavelength resolution have been transmitted farther than ever. Pavel Belov of Queen Mary University of London described his record-setting subwavelength transmission of light at the March Meeting. Imaging details smaller than half the wavelength of the light used to create the image has been a fundamental problem, but in the past few years scientists have been able to get around the classical diffraction limit with metamaterials, which have generated a lot of excitement lately in applications such as superlenses and cloaking. It’s a nice concept, but applications need improvement, Belov said. One problem is that although these materials can be used to create subwavelength images, these metamaterials materials can’t transmit an image very far.

Belov and colleagues used a new approach, using an array of parallel metal rods to channel light. The setup is similar in operation to a bundle of waveguides. They were able to reach about 1/20 wavelength resolution, and transmitted an image a distance of more than 3 wavelengths. So far, their experiments have used microwaves, but in simulations, their setup worked with wavelengths up to 30Thz, and in theory they believe it could work up to 100 THz. Belov suggested the technique could have applications, for example, in improving MRI resolution.